fix(toolchains): include libpython in python_repository attrs (#4382)

`_python_repository_impl` omitted `libpython` from its returned
reproducibility `attrs` dict, causing Bazel < 8.3.0 and non-`sha256`
invocations to emit a spurious `DEBUG` warning about dropping
`["libpython"]`.

Include `libpython` in `attrs` so Bazel preserves the attribute
without warning.

python/private/python_repository.bzl

diff --git a/python/private/python_repository.bzl b/python/private/python_repository.bzl
index d5ec92f..5a89553 100644
--- a/python/private/python_repository.bzl
+++ b/python/private/python_repository.bzl
@@ -322,6 +322,7 @@
         "coverage_tool": rctx.attr.coverage_tool,
         "distutils": rctx.attr.distutils,
         "distutils_content": rctx.attr.distutils_content,
+        "libpython": rctx.attr.libpython,
         "name": rctx.attr.name,
         "netrc": rctx.attr.netrc,
         "patch_strip": rctx.attr.patch_strip,